A year ago, when chemotherapy stopped working against his leukemia, William Ludwig signed up to be the first patient treated in a bold experiment at the University of Pennsylvania. Mr. Ludwig, then 65, a retired corrections officer from Bridgeton, N.J., felt his life draining away and thought he had nothing to lose.

Releasing genetically modified, spermless male mosquitoes into the wild could in future help to prevent malaria transmission and reduce the chances of large outbreaks of the killer disease, British scientists said on Monday.